1. Field of the Invention
The present invention relates generally to an apparatus and method for manufacturing sheet molding compound, and, more particularly, to an apparatus for manufacturing sheet molding compound while minimizing entrapped air therein.
2. Discussion of the Related Art
Compression molding is a technique for forming parts wherein a charge is placed between upper and lower heated die members defining a mold cavity. The dies are then brought to a closed position where the dies compress the charge causing it to flow and fill the mold cavity. After the resin cures, the molds are opened and the finished part is removed.
Compression molding techniques have been used to make parts having a relatively flat surface, such as exterior automotive body panels. The charges used for making such parts generally consist of a thermosetting resin containing reinforcing fibers and various fillers. Quite often, the charges are formed into sheets known in the art as sheet molding compounds (SMC). It has been difficult, however, to mold fiberglass reinforced plastic (FRP) parts so that they have an extremely smooth surface. The molded parts sometimes have surfaces that are blistered, rough or porous that require post-molding processing to smooth the blemishes. It is commonly believed that such blemishes result primarily from air that is trapped in the charge during molding. In an effort to minimize trapped air, one practice is to use a relatively thick charge which covers a relatively small area of the molding surface so that the air in the charge is "squeezed" out when the dies are closed.
It has also been recognized that the use of a vacuum during the compression molding process is useful in reducing the number of blemishes in the surface of the part. See, e.g., U.S. Pat. Nos. 4,488,862; 4,612,149; 4,551,085; and 5,130,071, which are hereby incorporated by reference. These prior patents teach applying a vacuum to the sheet molding compound charge during the compression molding process. Air also becomes entrapped, however, within the reinforcing fibers themselves while the fibers are mixed with the resin mixture paste during the manufacturing of the sheet molding compound charge. This trapped air can lead to voids or micro-pits on the surface of a molded part, requiring additional labor-intensive finishing procedures in order to produce a part with an acceptable surface thereby increasing manufacturing costs. There thus exists a need in the art to provide an improved method and apparatus for removing or minimizing entrapped air from the resin mixture paste and fiber reinforcing material while the sheet molding compound is formed in order to further reduce the occurrence of surface blemishes on molded articles.